1. Field of the Invention
The present invention relates to a method for producing butyraldehydes. Particularly, it relates to a method for separating and purifying mixed aldehyde products formed by hydroformylation of propylene into n-butyraldehyde and isobutyraldehyde by distillation.
2. Discussion of Background
A process which comprises hydroformylation of propylene to obtain mixed butyraldehyde products, separating the products by distillation into n-butyraldehyde and isobutyraldehyde, subjecting the obtained n-butyraldehyde to aldol condensation to obtain 2-ethylhexanal, which is further subjected to hydrogenation to obtain 2-ethylhexanol, or a process which comprises subjecting the n-butyraldehyde directly to hydrogenation to obtain n-butanol, has been world-widely practiced on an industrial scale.
As a conventional technique for separating butyraldehyde isomers, e.g. Japanese Unexamined Patent Publication No. 273841/1992 discloses a method wherein crude aldehyde products containing branched chain and straight chain aldehydes are distilled in a single distillation column to obtain three different product streams simultaneously. As the distillation conditions for this method, it is disclosed that the distillation column is operated at a temperature of from about 115.degree. to 140.degree. C. under such a pressure condition that the pressure at the top of the column is from 0.07 to 2.1 kg/cm.sup.2 G, and the Examples are carried out under a column top pressure of from 0.6 to 0.7 kg/cm.sup.2 G at a column bottom temperature of from 99.degree. to 129.degree. C., and the pressure at the bottom of the column is from about 1.08 to 1.42 kg/cm.sup.2 G as calculated from the vapor pressure of n-butyraldehyde.
Further, U.S. Pat. No. 5,227,544 discloses a method for producing 2-ethylhexanol of a high purity, which comprises adding a small amount of water to an aldehyde distillation column to hydrolyze an oligomer of isobutyraldehyde contained in the crude butyraldehyde and to recover its entire amount in the form of a monomer. Here, the temperature at the top of the aldehyde distillation column is described to be preferably from 70.degree. to 90.degree. C., and the pressure at the top of the column is from about 0.2 to about 1.3 kg/cm.sup.2 G, as calculated from the vapor pressure of isobutyraldehyde.
Such an aldehyde distillation column designed to separate n-butyraldehyde and isobutyraldehyde, requires a large plate number and a large amount of reflux, since the boiling points of aldehyde isomers to be separated are very close to each other, and reboiling is one of the sections which require the largest energy in the process.
Now, to discuss the heat balance of a process relating to unitization of exhaust heat, the process will be described as divided into a heat generating side and a heat consuming side.
Firstly, the heat generating side will be described with reference to literatures. INDICATIONS, Winter 1982/83 (The International Journal of Davy Mackee) discloses that a hydroformylation reaction is carried out at about 100.degree. C. under about 20 atm by maintaining a uniform catalyst solution in a reactor using an aldehyde polycondensation product as a solvent in the presence of a rhodium catalyst and a triphenylphosphine ligand.
Further, an Example of Japanese Unexamined Patent Publication No. 242038/1990 discloses a method wherein a reaction is carried out at 100.degree. C. in a completely mixing type reactor of a first stage using the same catalyst as used in INDICATIONS, and an unreacted gas is introduced in a bubbling column type reactor of a second stage to further conduct a reaction at 90.degree. C. An Example of Japanese Unexamined Patent Publication No. 112733/1985 discloses a method wherein a hydroformylation reaction is carried out at 120.degree. C. using a water-soluble rhodium-phosphine complex as a catalyst.
Furthermore, WO93/20034 discloses Examples wherein a condensation reaction of n-butyraldehyde using an aqueous NaOH solution as a catalyst, is carried out at a temperature of from 63.degree. to 120.degree. C., and Japanese PCT Publication No. 501483/1991 discloses Examples wherein a catalytic hydrogenation reaction of aldehydes is carried out at a temperature of from 120.degree. to 125.degree. C. Japanese Unexamined Patent Publication No. 39632/1983 discloses a gas phase hydrogenation process which is carried out at an outlet temperature of the reactor of 195.degree. C. However, in such a high temperature reaction, reuse of the heat of reaction is easy, and the heat of reaction is effectively used e.g. for preheating water for a boiler or a hydrogenation step.
Now, the heat consuming side will be described. The main parts in the process where the recovered heat is used, include, for example, a distillation column for alcohol purification and an aldehyde distillation column for separating a n-butyraldehyde and isobutyraldehyde. In the distillation column, reboiling belongs to the heat consuming side, and at the same time, the condenser at the top dissipates the heat and thus may be regarded as belonging to the heat generating side.
In the distillation for purifying 2-ethylhexanol, it is common to employ a column top pressure within a range of from a few tens mmHgA to about 100 mmHgA. The column top temperature is from about 100.degree. to 122.degree. C., as calculated from the vapor pressure, and the column bottom temperature is still higher by about 10.degree. to 30.degree. C. than the column top temperature. The distillation for purifying n-butyl alcohol is carried out usually under a column top pressure at a level of atmospheric pressure, the column top temperature is about 118.degree. C., and the column bottom temperature is still higher by about 10.degree. to 30.degree. C. than the column top temperature.
Thus, the temperature levels at the heat generating side and the heat consuming side are very close to each other, and it has heretofore been difficult to conduct heat recovery inexpensively by direct heat exchange.
The conventional aldehyde distillation column has been operated under a relatively high pressure condition as mentioned above in order to decompose and recover high boiling point substances in the mixed aldehyde products as valuable products and to increase the column top temperature by pressurizing to make the temperature difference from the cooling water large so as to reduce the heat conducting area of the column top condenser. Consequently, the column bottom temperature has been necessarily high, whereby the energy consumption at the time of reboiling is large, and it has been difficult to efficiently recover exhaust heat from other heat generating steps in the process.